The need for transmission systems in which the signals are optically transmitted to the terminal (TV receiver, home computer, etc.) is obvious today. One of the reasons is that in optical transmission systems, the signal bandwidth can be considerably wider than in electric transmission systems. Such an optical transmission system is described schematically in the journal "Funkschau", No. 22, 1990, page 56.
In this prior art system, no provision is made for monitoring the transmitted signal. However, network providers assume responsibility for the quality of the signal up to a given point in the transmission link. They therefore require the network manufacturer to provide a facility which makes it possible to monitor a signal at this point (henceforth called "monitoring point").
It is the object of the present invention to provide a system which permits such signal monitoring.
This object is attained by a system for optically transmitting a useful signal from a transmitter over an optical waveguide (3) or a fiber-optic network to a receiver, characterized in that in addition to the useful signal (S.sub.1), the transmitter (1) transmits an auxiliary signal (S.sub.2), that the useful signal and the auxiliary signal are transmitted over the same optical waveguide (3) or the same fiber-optic network toward the receiver (2) at different wavelengths, that at a desired point in the transmission link, means (4a, 4b, 5, 6, 7) are provided for extracting and analyzing the auxiliary signal (S.sub.2) and for transmitting the results of the analysis, if necessary together with other information, as a response signal (S.sub.3) over the same optical waveguide (3) or the same fiber-optic network to the transmitter (1), and that the transmitter includes means for controlling the further transmission of the useful signal and the auxiliary signal in accordance with the response signal.
A further advantageous feature of the invention is that before the desired point the useful signal (S.sub.1) and the auxiliary signal (S.sub.2) pass through a fiber-optic amplifier (9) comprising a pump source (7), and that the pump source (7) also serves to transmit the response signal (S.sub.3).
An additional feature is that the wavelength at which the auxiliary signal (S.sub.2) is transmitted and the laser of the pump source (7) are so chosen that the auxiliary signal can be passed through the laser without being changed and without interfering with the operation of the laser.
In conventional attempts to attain these objects, it is necessary to convert the optical signal to an electric one. Such a proposal is described in Gunter Domann, J. of Lightwave Technology, Vol. 6, 1988, pages 1720-1727.